Interstellar dust in nebulae and in the diffuse interstellar medium of galaxies contains a component which responds to illumination by stellar photons with efficient luminescence in the 500 nm to 1000 nm spectral range, known as extended red emission (ERE). The carrier of the ERE remains unidentified, although it must provide a substantial fraction of the total UV/optical absorption cross section of interstellar dust. Here we attempt to constrain the nature of the ERE carrier by investigating the wavelength region in which the ERE carrier absorbs radiation rather than by trying to match its emission spectrum. Our method relies upon the wavelength dependence of extinction by dust in dense molecular clouds, which permit UV/optical radiation of different wavelengths to penetrate to different depth within the cloud. Using HST observations of photo-dissociation filaments seen edge-on in the reflection nebula NGC 7023, we find that the radiation that initiates the ERE penetrates into molecular cloud surfaces only slightly more deeply (by ≈ 12%) than does the radiation in the Lyman and Werner bands of H2 (λ ≤ 1104 Å), which excites the near-IR 1-0 S(1) line emission of H2. This implies that far-UV photons with energies > 10 eV are required to initiate the ERE. None of the existing models for the ERE carrier, which are based upon photo-luminescing agents with band gaps of order 2 eV, can meet this new requirement. One possible interpretation of our new finding is that ERE is produced by its carrier only after it is photo-ionized (once or twice) and then pumped by optical radiation. This research was supported by the NSF through Grant AST 0307307 to The University of Toledo. Support for program #9741 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.